Are you running Windows 7 64-bit? Looks like this is a fairly common issue although I seem to remember it working fine for me with the same OS. The fix is to set hl2.exe to run in XP compatibility mode - go to the installation directory of thre game, right click hl2.exe, go to properties, compatibility tab, tick the first box and select Windows XP Service Pack 3. Hopefully this will resolve the issue.